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VLA Test Memorandum No. 202:
Referenced Pointing at the VLA
Michael P. Rupen
9 January 1997
This memo is the original version of a Web page devoted to referenced
pointing. That Web page will change to track the best current information;
the purpose of putting the original out as a memo is to keep some more
permanent record not subject to the vaguaries of Web maintenance.
Using the standard pointing constants, the pointing error for the
VLA antennas is generally 10 to 20 arcseconds, and can be as bad as an
arcminute. These pointing errors significantly degrade the sensivity at
the nominal field center at the higher frequencies, for which the a
priori pointing can be off by a large fraction of the primary beam (FWHP
about an arcminute at 7mm (Q band) and two arcminutes at 1.3cm (K band)).
Single dish observers habitually "fine tune" the pointing near a source
by pointing up on a nearby bright object, a procedure involving measuring
the signal at the nominal position and four positions offset in each of the
cardinal directions (N S E W), and using a simple model for the beam shape to
find the peak in the received signal. A similar procedure is now available for
the VLA; here it is referred to as referenced pointing.
The purpose of this memorandum is to collect the available information
and lore about referenced pointing, and to communicate a simple
understanding of why, when, and how to use it. The references at the end
give more detailed information.
Contents:
The basic procedure for using referenced pointing is simple:
- determine the pointing offsets by pointing up on
a nearby, bright, point-like source;
- tell the on-line system to apply those corrections to any number
of subsequent scans;
- go back to (1) when the pointing has changed significantly from
when the pointing offsets were determined. The pointing may change
for example because the source has moved to a different (Az, El), or
because a change in temperature has slightly altered the shape of the dish.
The pointing offsets are usually determined using full-bandwidth continuum
observations at 3.6cm (X band): full bandwidth to give maximum sensitivity;
3.6cm both because that is the most sensitive band at the VLA, and because
the a priori pointing even in the worst cases will lie well within the
half-width of the beam at that frequency (about 2.7 arcminutes). The pointing
calibrator should be near where the source will be, during the time for which
the derived pointing offsets will be used; it should be bright, so that the
pointing observations can be short without thermal noise dominating the
solutions; and it should be point-like, so that source structure does not get
confused with the antenna response. The actual limits depend on the
sensitivity and pointing behavior of the VLA. How often to check the
pointing depends on the quality of the default pointing model, and on the
local weather conditions (e.g., sunrise and sunset lead to significant
temperature gradients which can produce rapidly varying pointing errors).
The current wisdom on these issues is given
in the section on Lore, below.
Notice that the pointing is determined at X band, and is then applied to
scans at arbitrary observing frequencies. This is called "primary
referenced pointing", to distinguish it from "secondary referenced pointing"
(see the next section), in which an additional pointing calibration is done
at the same frequency as the main observations.
Second order referenced pointing, also referred to as "double referenced
pointing" (e.g., in the 7mm Status Summary) or "secondary referenced pointing"
(e.g., in OBSERVE), was developed to refine the pointing corrections even
further. Primary referenced pointing determines primary pointing offsets
based on observations at some relatively low frequency, usually X band;
secondary referenced pointing determines
additional, presumably smaller secondary pointing offsets, based on
observations at the frequency of interest. There are two reasons this might be
useful. First, there may be small differences in the pointing
(because of collimation differences) between the bands. Second,
the smaller primary beam at higher frequencies can give more accurate
pointing determinations.
The procedure is an elaboration of that for primary referenced pointing:
- determine the primary pointing offsets at X band;
- determine the secondary pointing offsets at the desired frequency;
- tell the on-line system to apply both primary and secondary corrections
to any number of subsequent scans on calibrator(s) and source(s) of
interest;
- if you wish to touch up the pointing after a while but feel the
primary pointing offsets are still good enough, return to (2);
- if you wish to touch up the pointing after a while and feel the
primary pointing offsets are not still good enough, return to (1).
Second order referenced pointing is nicely adapted to observations of a
single source at multiple frequencies. You might for instance determine
primary pointing offsets at X band, determine secondary offsets at Q band,
observe the source at Q band, determine secondary (refresh) offsets at K
band, observe at K band, determine secondary (refresh) offsets at U band,
observe at U band, etc.
We don't know yet whether second order works better than primary referenced
pointing alone, or (if it does) how often one needs to update the secondary
and the primary pointing offsets.
- Pointing accuracy, before and after:
- Without referenced pointing, the "blind" rms pointing errors
(averaged over antennas and the sky) are about
15-18 arcseconds under good weather conditions on calm nights (Newell 1983,
Morris 1991). The daytime rms pointing errors are probably at least 20
arcseconds (Morris 1991; but see also Newell 1982).
Drifts of up to 35-40 arcseconds, on timescales of less than 30 minutes,
have been reported around sunrise and sunset (e.g., Bagri 1996).
Errors on individual antennas, in some parts of the sky, can be up to
about an arc minute.
Primary referenced pointing (when successful) lowers the rms
pointing error to about 2 arcseconds in azimuth and 5 arcseconds in
elevation, for elevations below about 80 degrees (R. Perley, M.S. Yun,
priv. comm.).
Above 80 degrees the pointing changes rapidly with sky position, making
referenced pointing less effective. Why this is, and whether it can be
corrected, is currently under study.
No firm results are available for the efficacy of secondary referenced
pointing. Bagri (1996) showed that there is no systematic offset
between the pointing at Q, K, and X bands.
- When to use referenced pointing:
- Referenced pointing should be used if the improved sensitivity (due to
accurate pointing) outweighs the time lost to pointing calibration.
Primary referenced pointing is essential at Q band (7mm), and probably
helps at K band (1.3cm) as well. At U band (2cm) the improved sensivity
roughly cancels the time spent pointing, while pointing at the longer
wavelengths is generally a waste of time. The NRAO staff are still
debating whether secondary referenced pointing is ever worth doing, even
at 7mm.
Another case where referenced pointing may be helpful is that of high
dynamic range mosaicing. Consider a source like Centaurus A, dominated
by a very bright central source but with interesting, low-level extended
structure. In mosaicing one observes several pointings separated by
something like the half-width of the beam; this means that the points
adjacent to the center would have the bright central source roughly at
the half-power point. Obviously it would be useful to have all the
telescopes pointed well enough that their half-power points roughly
coincided, so that the relative gains of the antennas were roughly the
same both at this very bright source way out in the sidelobe, and near
the center of the beam where you're actually pointing. Otherwise
the equivalent of calibration artifacts from the bright source will
spread junk over the rest of the field. Referenced pointing would
alleviate the problem by ensuring that the gains of all the antennas
varied in as similar a fashion as possible across the primary beam.
- How often to point up:
- As a rough guide, pointing up every hour or so seems adequate,
except when the temperature is changing rapidly. The most predictable
instances of the latter are sunrise and sunset, around which times it
is advisable to point up every half hour. See e.g. Bagri (1996) for
an example of the data leading to these conclusions.
We do not yet know how often secondary referenced pointing should be
done -- if it should be done at all.
- Flux density required for pointing calibrator:
- For successful pointing calibration at X band using the standard
10 second integrations, the pointing calibrator should be at least 0.3 Jy.
Stronger is better for flux densities up to about 1 Jy; for even stronger
sources, thermal noise no longer dominates the errors in the solutions.
Longer integration times do increase the sensitivity as sqrt(t),
assuming the phases are coherent on those timescales; shorter
integration times similarly decrease it.
The required flux density for the pointing calibrator should scale
roughly linearly with the VLA's sensitivity and the size of the beam,
but this has not yet been measured. For now one may probably safely
assume minimum flux densities of about 1 Jy at 2, 1.3, and 0.7cm.
Probably this is a bit conservative at the longer wavelengths.
Note that the source must be this strong on individual
baselines! The VLA determines pointing corrections using
amplitudes calculated from self-calibration of the interferometric
visibilities, using a point-source model. A resolved source will work
less well than one which is truly point-like.
- Distance from pointing calibrator:
- For pointing calibration to be useful, the pointing corrections must
be nearly the same for the pointing calibrator and for the source(s) to
which the corrections will be applied. M.S. Yun's analysis of the regular
pointing runs suggests that the pointing calibrator should be no more than
10 degrees in Az or El from the source position(s); azimuth seems to be
more critical (Yun 1997). There is anecdotal evidence for
up to 30arcsec pointing errors near transit, even when the pointing
calibrator is within 10 degrees of the source (M.S. Yun, priv. comm.).
Clearly putting the pointing calibrator and the observed source(s) on
opposite sides of the zenith is a bad idea.
- Size required for pointing calibrator:
- The source used for pointing calibration should obviously be small
compared to the primary beam, to avoid confusing pointing errors with
source structure. A more stringent limit is that there be
enough flux on individual baselines to allow ANTSOL to find a
solution; and that any structure larger than the synthesized beam should
not cause significant variations in the gains reported by ANTSOL on the
timescale of the pointing scan (a few minutes). These considerations
imply that VLA phase calibrators suitable for the array and wavelength in
question will be the most reliable pointing calibrators as well.
- Pointing at high elevations:
- The pointing model for the VLA is particularly bad at elevations
above 80 degrees (Yun 1997), and the residual pointing errors change
rapidly with
sky position there as well. Furthermore diurnal source motion near
zenith can lead to rapid changes in azimuth as the source transits,
again giving large differences in the residual pointing errors from one
moment to the next. Both the a priori and referenced pointing
therefore do a very poor job at high elevations, though the errors made
have not yet been quantified.
- Effect of wind on pointing:
- 8 m/s (18 mph) winds give an additional pointing error of 23 arc
seconds for 90% of the sky (Morris 1991). In one test run (R. Perley,
priv. comm.), pointing calibration failed (no solutions for more than
half the antennas) for winds this strong or stronger. Clearly one
should not bother with referenced pointing in these conditions, and
should also not observe at all at 7mm (Q band). The problem with 7mm
observations in particular is that, with the antennas flopping about
in the wind, some will be seeing the source not at the beam center but
off at the half-power point; as the pointing moves around the observed
amplitude of the source will change rapidly, by large factors.
In 6-8 m/s winds, that same test run found that pointing calibration was
iffy -- sometime it worked, sometimes not. Pointing in slightly less
strong winds worked as well as in completely calm conditions (R. Perley,
priv. comm.).
Note that the wind speed is reported occasionally in the operator's log
(automatically sent to the observer), and stored for each pointing scan in
a computer file (see Finding Out What's Going On,
below, to find out how to get this).
- When pointing calibration fails:
- During a pointing scan the pointing offsets are calculated by the
on-line system and stored for use in future scans. Sometimes no good
solutions will be found for one or more antennas. In these cases the
pointing offsets for those antennas are set to zero. Primary and
secondary pointing offsets are stored separately. It is quite possible
for a secondary pointing scan to fail after primary pointing calibration
has yielded good solutions; on such occasions the primary pointing offsets
will be left as they were (i.e., non-zero), while the failed secondary
pointing offsets will be zeroed.
This section is based primarily on conversations with and memos from Ken
Sowinski. The latter are listed in the section on
references. The "What actually happens" sections
are essentially a rehash of Sowinski's memos; they are included here for
completeness, and as background for the remainder of this document.
Whenever a subarray is in pointing mode, a program (PTG) is run to
estimate the current pointing offsets for each antenna.
The pointing scan itself consists of a series of five-point pointing
cycles. The phase center of the array is kept constant at the nominal
position of the calibrator. For each of the five positions, ANTSOL
derives the antenna gains via self-calibration of the interferometric
visibilities using a point-source model. The amplitude gains from the five
ANTSOLs are collected by PTG, which derives the beam width and pointing
offset in Az and El for each antenna.
The offsets are considered valid and recorded only if three conditions are
met:
- There must be enough flux at each pointing, as reported for each
antenna by ANTSOL, to be able to determine a statistically significant
solution.
- The offset and beamwidth that are estimated must be reasonable.
- There must be valid solutions for both polarizations, since the
offsets stored are the average of the offsets determined for each
polarization.
This last condition is important if the pointing scan is done in spectral
line mode: a correlator mode must be chosen that will provide at least one
IF of each polarization.
The resulting beamwidths and offsets are recorded
(1) in a file which may be processed off-line to determine updates to
the default pointing model;
(2) on paper, for inspection in real time (note that less information is
reported here);
and
(3) in memory, so the offsets may be used to correct the pointing model
for subsequent scans.
Currently, by default, none of these records are actually sent
to the observer. Eventually the procedure will be streamlined. For now,
the paper versions may be obtained by putting a
request under the Special Instructions section of the OBSERVE file; these
will then be mailed to the observer after the run. Similarly, the observer
can obtain an ASCII summary of the pointing file (with rather more
information) by requesting this before the observing run, through
the analysts
(analysts@nrao.edu)
or one of the support staff. See Appendix 1
for a description of this file.
The dwell time for a pointing scan must be long enough to allow at least one
five-point cycle (no less than two minutes when using a ten second integration
time). The integration time should be a multiple of ten seconds:
shorter integration times will only decrease the sensitivity of the
observations, not save any time, as each point in the cycle will still
take 10 seconds on-source. (There are still some special cases where shorter
integration times might be desirable; for instance, if the weather is so bad
that the phase wraps in 10 seconds prevent ANTSOL from finding a solution.
Normally one would just give up at that point, but if one's source is
sufficiently strong for self calibration on those timescales, one can still
obtain useful data.)
If the source used for a pointing scan is not a calibrator (i.e.,
the "calcode" is blank), it is forced to be a calibrator (it is given
calcode 'P') so that ANTSOL will run, and we hope for the best. See the
section on Lore for a discussion of what sort of
sources can be used as pointing calibrators.
Second order referenced pointing is described fully in Sowinski's 1996
screed,
Second Order Referenced Pointing. Smudging over a few details,
it works basically as above, except that the results are stored in a separate
table in memory.
The complications of second order referenced pointing within OBSERVE files
are discussed below.
The pointing offsets that have been determined will be applied (1) if
you ask for it, (2) if they exist, and (3) if they are not more than 12
hours old. (There is also a position test, but that is currently disabled.)
These tests are applied independently for each antenna in the
subarray. When referenced pointing is requested a bit is recorded on the
Archive tape which describes, for each antenna, whether or not the pointing
correction was applied. This bit knows only about the last pointing scan, and
so will refer to the last secondary/refresh pointing scan in the case of
second order referenced pointing. FILLM (as of 15OCT96) does not yet do
anything with this information.
Asking for pointing offsets to be applied during a primary pointing scan is
generally a bad idea, as the new pointing offsets determined in
that scan will not be rememebered. Occasionally this mode may be useful,
for instance to gauge how well referenced pointing is working, or to
estimate interband collimation errors. Secondary pointing scans by definition
use the existing pointing offsets.
The source card contains all information on referenced pointing.
- Columns 59-60 give the Observing Mode:
- "IR" --> a pointing scan; could be either primary or secondary.
In OBSERVE, set by ObservingMode = "Det. primary ref pntg",
"Det. secondary ref pntng", or "Det. refresh ref pntng".
- "IA" --> identical to "IR", but sends more output to the line
printer at the VLA.
In OBSERVE, set by ObservingMode = "Interf. pointing, I.F. A".
- " " --> regular scan. In OBSERVE, set by ObservingMode "Standard
Interferometer".
There are various other possibilities as well, but these are the ones
most important for referenced pointing.
- Column 69 contains a flag determining whether previously determined
offsets are applied. This flag has slightly different implications
depending on whether this is a pointing scan, as determined by the
Observing Mode entry described above.
Standard observations (Obs. Mode " "):
- blank --> do not apply any previously determined offsets.
In OBSERVE, set by Referenced Pointing = blank or No.
- T --> apply previously determined offsets; if both primary
and secondary offsets are available, their sum will be used.
In OBSERVE, set by Referenced Pointing = Yes.
Pointing scans (Obs. Mode "IR" or "IA"):
- blank --> do not apply any previously determined offsets;
any previously determined primary and secondary offsets will be
forgotten; the offsets determined during this scan will be
remembered. Should be used for primary referenced pointing.
In OBSERVE, set by Referenced Pointing = blank or No.
- T --> apply previously determined offsets; the offsets
determined during this scan will NOT be remembered. This mode
should NOT be used for ordinary pointing scans!
In OBSERVE, set by Referenced Pointing = Yes.
- S --> previously determined offsets will be applied and saved
as "primary" offsets; the offsets determined during the scan will be
saved as "secondary" offsets. Should be used for the first
secondary pointing determination following a primary pointing
determination. In OBSERVE, referred to as ObservingMode
"Det. secondary ref pntng".
- R --> any previously saved primary offsets will be applied
and not changed; previously determined secondary offsets are
forgotten; the offsets determined during this scan will be saved as
secondary offsets. Should be used for the second and subsequent
secondary pointing determinations following a primary pointing
determination. In OBSERVE, referred to as ObservingMode
"Det. refresh ref pntng".
Note that there is a subtle distinction between the column 69 flags "S" and
"R". The former saves the previous offsets as "primary" offsets; the latter
does not. This means that there should always be a mode "S" scan before any
mode "R" scans. This is illustrated in the sample OBSERVE
files, below.
OBSERVE version 3.2.0 is the earliest version which knows about primary
referenced pointing; version 4.0 is the earliest version which knows about both
primary and secondary referenced pointing. The latest version of OBSERVE
is available via anonymous ftp
(ftp ftp.aoc.nrao.edu, and look in pub/observe). If you have difficulty
fetching or installing it, please send email to Wes Young
(wyoung@nrao.edu).
The following discussion is based on OBSERVE 4.0. Earlier versions are
similar but did not know about secondary/refresh referenced pointing.
Within OBSERVE, referenced pointing is handled on the Source page. The
two important entries are (1) ObservingMode, and (2) Referenced Pointing.
ObservingMode determines whether this is a pointing scan or not; Referenced
Pointing determines whether any pointing offsets previously found are
applied.
ObservingMode "Det. primary ref pntng" should be used for primary
pointing scans; ObservingMode "Det. secondary ref pntng" or "Det. refresh
ref pntng" should be used for secondary pointing scans. "Det. secondary
ref pntng" sets the referenced pointing flag in column 69 to "S"; "Det.
refresh ref pntng" sets that flag to "R". The difference between those two
flags is discussed in the previous section -- basically the first secondary
pointing scan following a primary pointing determination should use "Det.
secondary ref pntng", while all subsequent secondary pointing scans should
use "Det. refresh ref pntng".
Referenced Pointing can be set either to Yes or No (blank means No).
If you want referenced pointing corrections applied to any regular
(non-pointing) scan, set this to Yes. Otherwise, set this to No. The
most common mistake is to set Referenced Pointing Yes for pointing scans:
this will not affect ObservingMode "Det. secondary... or "Det. refresh
ref pntng" scans, but will make ObservingMode "Det. primary ref pntng" scans
useless.
The section on sample OBSERVE files gives examples
of runs involving referenced pointing.
This section collects various important but arcane restrictions. See
also the section on Lore for more detailed comments.
Please let me know if you run across restrictions or problems not duscussed
here!
Pointing scans:
- Correlator mode:
must use a mode which provides at least one IF of each polarization --
even if the rest of the experiment uses only a single polarization.
For primary referenced pointing one usually uses the NRAO Default XX,
i.e., standard continuum observations at X band.
- Integration time:
should be a multiple of ten seconds for pointing scans. Can be shorter
if necessary due to rapid phase wrapping.
- Dwell time:
should be enough to allow at least one full five-point pointing cycle:
two minutes minimum, when using 10 second (or shorter) integration times.
- Calibrator type:
need not be a calibrator, but ANTSOL must give results indicating
sufficient flux for statistically significant pointing solutions.
VLA calibrators are probably the safest bets, with minimum flux
densities of about 0.3 Jy at X band (3.6cm), and 1 Jy at shorter
wavelengths.
- Applying pointing offsets to a pointing scan:
DON'T! unless you really know what you're doing.
- A mode "S" pointing scan determines secondary pointing offsets, and
should occur after a primary pointing scan. If several secondary
pointing scans are to be done between primary pointing scans, the first
must be mode "S", and all others must be mode "R".
Referenced pointing must be turned on in all "regular" scans, in order to
use the offsets determined during pointing scans. Otherwise the a
priori pointing will be used.
Spectral line pointing and referenced pointing in multiple subarrays
have various other oddities as well -- these are covered in the section on
Special Cases, below.
- A Simple File
-
We want to observe a single source at K band, using referenced pointing
for maximum sensitivity. We don't think secondary referenced pointing
is worth it. We want to use 5 second integrations for our observations.
The run might be sketched as follows:
- Primary pointing scan on flux cal. at X band: 2min10sec dwell time
- Flux cal. at K band: 3min dwell
- Primary pointing scan on phase cal. at X band: 2min10sec dwell
- Phase cal at K band: 1min dwell
- Source at K band: 9min dwell
- Phase cal at K band: 1min dwell
- Source at K band: 9min dwell
- etc.
The corresponding OBSERVE file, using default frequencies and starting
on 3C286 at 16:00, would be:
/.ADA000
//* ***
//* *** NRAO VLA Observe Program, Version U4bbbbb, 1996.12.29
//* ***
//* *** Observation day 57,160 at 16 00 00 LST, 1997.01.02 09:21:06 MST.
//* ***
//* *** Observer
//* *** I. Point-Well Phone
//* *** Office: 505/835-6666
//* *** During observation:
//* *** E-Mail address
//* ***
//* *** Observing mode(s): Continuum
//* ***
//* *** Special Instructions
//* *** Please send hardcopy of pointing results to the observer.
//* ***
//* *** Date Prepared: 1997.01.02 10:47:03 MST.
//* ***
//* Primary pointing scan: flux cal
1328+307 16 03 11 13 28 49.6579 +30 45 58.641 XX IRA 0000
//DS 10
//* Flux cal
1328+307 16 06 34 13 28 49.6579 +30 45 58.641 KK A 0000T
//DS 5
//* Primary pointing scan: source/phase cal
1624+416 16 11 51 16 24 18.2505 +41 41 23.552 XX IRA 0000
//DS 10
//* Phase cal
1624+416 16 13 16 16 24 18.2505 +41 41 23.552 KK A 0000T
//DS 5
//* Source
N6145 16 22 38 16 23 21.6000 +41 03 35.000 KK 0000T
//DS 5
//* Phase cal
1624+416 16 24 04 16 24 18.2505 +41 41 23.552 KK A 0000T
//DS 5
//* Source
N6145 16 33 29 16 23 21.6000 +41 03 35.000 KK 0000T
//DS 5
//* Phase cal
1624+416 16 34 58 16 24 18.2505 +41 41 23.552 KK A 0000T
//DS 5
//* Source
N6145 16 44 23 16 23 21.6000 +41 03 35.000 KK 0000T
//DS 5
etc.
-
-
Notes:
- The pointing scans use 10 second integrations, while
the ordinary scans can use whatever is desired (in this case, 5
seconds).
- The pointing scans do NOT have a T after the bandwidth code:
referenced pointing is turned OFF during primary pointing scans.
- The pointing scans have dwell times (taking data) of 2min10sec,
a bit longer than the required 2min just to be certain.
- The "special instructions" section must specifically ask for
pointing results to be sent to the observer; otherwise the printed
results will vanish, at least from the observer's perspective.
- Secondary Referenced Pointing
- Let's say we want to map the continuum from 3C84 at 7mm, over the
entire VLA primary beam. We want to do secondary referenced pointing,
in hopes that will help the 7mm sensitivity, and because the field
is large enough that it matters whether the half-power points of the
different antennas' primary beams coincide.
We also want to be sure that the phase does not wrap significantly
during an integration, so we use 3 second
integrations throughout, even for the pointing scans (otherwise ANTSOL
might fail due to the phase wrap).
The plan then is something like
- Primary pointing scan on flux cal. at X band: 2min10sec dwell time
- Secondary pointing scan on flux cal. at Q band: 2min10sec dwell time
- Flux cal. at Q band: 3min dwell
- Primary pointing scan on source at X band: 2min10sec dwell
- Secondary pointing scan on source at Q band: 2min10sec dwell
- Source at Q band: 10min dwell
- Refresh pointing scan on source at Q band: 2min10sec dwell
- Source at Q band: 10min dwell
- Refresh pointing scan on source at Q band: 2min10sec dwell
- Source at Q band: 10min dwell
- etc.
The corresponding OBSERVE file, using default frequencies and starting
on 3C84 at 03:00, would be:
/.AP0001 115
//* ***
//* *** NRAO VLA Observe Program, Version U4bbbbb, 1996.12.29
//* ***
//* *** Observation day 57,160 at 03 00 00 LST, 1997.01.01 20:23:14 MST.
//* ***
//* *** Observer
//* *** Rick Perley Phone
//* *** Office: 505-835-7312
//* *** During observation: 505-835-3317
//* ***
//* *** E-Mail address
//* *** rperley@nrao.edu
//* ***
//* *** Observing mode(s): Continuum
//* ***
//* *** Special Instructions
//* *** This file is for Subarray #1. Put the Q-band antennas in this
//* *** subarray. Ensure the first pointing scan completes before progressin
//* *** Please send all printouts to Rick Perley.
//* ***
//* ***
//* *** Date Prepared: 1997.01.02 14:30:05 MST.
//* ***
//* Primary pointing scan for 3C84
0319+415 03 05 00 03 19 48.1601 +41 30 42.106C XX IRA 0000 24.00
//DS 3
//* Secondary pointing scan for 3C84
0319+415 03 07 30 03 19 48.1601 +41 30 42.106C QQ IRA 0000S
//DS 3
//* source scan on 3C84
0319+415 10 03 17 50 03 19 48.1601 +41 30 42.106C QQ A 0000T
//DS 3
//* Refresh pointing scan on 3C84
0319+415 03 20 20 03 19 48.1601 +41 30 42.106C QQ IRA 0000R
//DS 3
//* source scan on 3C84
0319+415 20 03 30 40 03 19 48.1601 +41 30 42.106C QQ A 0000T
//DS 3
//* Refresh pointing scan for 3C84
0319+415 03 33 10 03 19 48.1601 +41 30 42.106C QQ IRA 0000R
//DS 3
//* source scan on 3C84
0319+415 30 03 43 30 03 19 48.1601 +41 30 42.106C QQ A 0000T
//DS 3
//* Primary pointing scan for 3C84
0319+415 03 46 00 03 19 48.1601 +41 30 42.106C XX IRA 0000 24.00
//DS 3
//* Secondary pointing scan for 3C84
0319+415 03 48 30 03 19 48.1601 +41 30 42.106C QQ IRA 0000S
//DS 3
//* source scan on 3C84
0319+415 10 03 58 50 03 19 48.1601 +41 30 42.106C QQ A 0000T
//DS 3
//* Refresh pointing scan on 3C84
0319+415 04 01 20 03 19 48.1601 +41 30 42.106C QQ IRA 0000R
//DS 3
//* source scan on 3C84
0319+415 20 04 11 40 03 19 48.1601 +41 30 42.106C QQ A 0000T
//DS 3
//* Refresh pointing scan for 3C84
0319+415 04 14 10 03 19 48.1601 +41 30 42.106C QQ IRA 0000R
//DS 3
//* source scan on 3C84
0319+415 30 04 25 00 03 19 48.1601 +41 30 42.106C QQ A 0000T
//DS 3
etc.
-
-
Notes:
- The pointing scans use 3 second integrations to allow for very
rapid phase wraps at 7mm. The scan still has to last at least 2mins.
Using 3 second averages gives lower sensitivity than the usual 10
seconds, but 3C84 is ridiculously strong (24 Jy at 3.6cm, 10.5 Jy at
7mm) so it doesn't really matter.
- Note the use of secondary and refresh referenced pointing, as
seen in the use of "S" and "R" after the bandwidth code on the source
cards. Secondary ("S") pointing scans occur just after primary
pointing scans, with refresh ("R") pointing being used for subsequent
pointing updates. There is probably no need to do another primary
pointing at all, given how often secondary/refresh pointing is done;
here the observer is being somewhat cautious.
- As above, the "Special Instructions" section asks explicitly that
pointing results be sent to the observer. Here the observer has
also requested that the first scan be extended as long as necessary to
get a good pointing solution; otherwise, if the previous observer left
the array 180 degrees from 3C84, the first pointing might never get done,
which would probably lead to the secondary pointing failing as well.
As mentioned above, various subsets of the pointing scan results are kept
in three places: on paper at the VLA site; in a file on one of the online
computers; and in computer memory for use when requested by subsequent scans.
Currently one must be in New Mexico to see the pointing solutions
in real time. If you want to poke around from Socorro, consult one of the
local pundits (see the section on Help!).
At the VLA site, the number of antennas which fail to point up is
printed out in real time for each pointing scan, so you have some idea how
you're doing, but not what the actual pointing offsets are.
Using observing mode IA, the pointing offsets and beam sizes are also
printed.
If you're
pointing up in several subarrays at once this can get rather confusing...
After the observations, the printouts from the site will be mailed to
the observer, if that has been requested in the Special Instructions section
of the OBSERVE file. A text version of the computer file can also be sent;
this should be requested well before the observing run, by
emailing the analysts
(analysts@nrao.edu)
or one of the support staff. A sample of this computer listing is given in
Appendix 1. Basically it gives the numbers of
antennas which did and did not point successfully; the beamwidths and
pointing offsets derived for those which did; and a few useful tidbits about
the weather, including the wind speed.
Eventually FILLM should be modified to notice the referenced pointing
flag bit set by the online system, but this is a ways off yet. At some
point the online system will also be modified to make the pointing results
easier to access; this again will be a while.
- Multiple subarrays:
pointing simultaneously in multiple subarrays now works. However,
there are still a few hazards.
- The first subarray determines integration times; so if you're
pointing in subarray 2, you should be sure the integration time in
subarray 1 is a multiple of 10 seconds.
- The correlator requires that all subarrays must be in continuum,
or all subarrays must be in spectral line modes. This may
force you to point up in spectral line mode. Note that there are
some odd correlator restrictions on bandwidths in the various
subarrays -- see Sowinski's memo on
Some Issues for Q Band Observing.
- Spectral line pointing:
this works, but has seldom been used. Contact the analysts
(analysts@nrao.edu)
before doing anything rash! In general pointing is better done in
continuum mode.
- Pointing scans require good results from both
polarizations. Any spectral line mode used for pointing must include
at least one IF of each polarization.
- A pointing scan may use different bandwidths for the different IFs.
- In spectral line mode, the pointing solutions will be based on
the "Channel 0" data, corresponding to the inner three-quarters of
the requested total bandwidth. Obviously this means the pointing
calibrator has to be even stronger than usual. One cannot
request that pointing be done using a particular channel, e.g. for a
maser source; at best one can only narrow the total bandwidth
as much as possible.
- There is a factor two difference in the required flux levels for
spectral line and continuum pointing, in the sense that the source
flux must be a factor two stronger to successfully point up
in spectral line mode. Since the sensitivity will also be lower,
due to the decreased bandwidth, pointing calibrators must be quite
strong to be useful in spectral line mode.
- VLBI:
referenced pointing can certainly be used in VLBI experiments, and
at the highest frequencies undoubtedly should be.
- Walker's sched program knows about primary referenced pointing
and applying pointing offsets; the relevant commands are PEAK and
NOPEAK. See the sched manual for details.
- PTG also has a "2-antenna mode" that allows pointing corrections
for VLBI observations using only a single VLA antenna. This is
invoked just like primary referenced pointing, but for a
single-antenna subarray. Since the VLA points up using
interferometric data, a reference antenna is "stolen" from the main
array, and the pointing is done based on the change in amplitudes on
that one baseline as the VLBI antenna moves through the five points.
Talk to a pundit for details.
For more detailed information or more lucid explanations, talk to
Chris Carilli
(ccarilli@nrao.edu),
Michael Rupen
(mrupen@nrao.edu),
or Greg Taylor
(gtaylor@nrao.edu).
This document is based on discussions with local pundits, particularly
Bryan Butler, Chris Carilli, Frazer Owen, Rick Perley, Bill Sahr, Ken Sowinski,
Wes Young, and Min Su Yun; as well as on the references given below.
- Bagri, D.S. 1996. VLA Test Memo No. 199: Antenna Pointing.
- ...test observations of VLA and VLBA pointing variations in Nov-Dec
1995
- Carilli, C. 1996.
VLA 7mm System Status.
- ...includes hints and suggestions for 7mm pointing, and a sample
OBSERVE file
- Kestevan, M. 1994.
VLA Test Memo No. 183: Reference Pointing and the Pointing
Mode.
- ...mostly deals with the (old) pointing model; some discussion of
referenced pointing tests, and whether second order referenced
pointing is necessary
- Newell, R.T. 1982. VLA Test Memo No. 138: Antenna Thermal
Insulation.
- ...gives day/night pointing results for two antennas
- Newell, R.T. 1983. VLA Test Memo No. 142: Discussion of VLA
Pointing.
- ...review of pointing procedures and results from the good ol' days
- Morris, D. 1991. VLA Test Memo No. 182: A Review of VLA
Pointing.
- ...summary of "blind" pointing characteristics; analysis of sources
of errors, with particular attention to the effects of wind
- Sowinski, K. 1995.
Some Issues for Q Band Observing.
- ...includes the basic discussion of how to use referenced pointing
- Sowinski, K. 1996a.
Second Order Referenced Pointing.
- ...the basic discussion of second order referenced pointing;
includes very clear explanations of what the OBSERVE cards actually
mean
- Sowinski, K. 1996b.
VLA Online Software Update of December 4, 1996.
- ...corrected a few bugs in pointing calibration
- Yun, M.S. 1997.
VLA Pointing Update.
- ...a description of the improved VLA pointing model, implemented
in November 1996, and the resulting pointing errors as functions of
(Az, El). To be released Real Soon Now.
The text printed out in real-time at the site simply reports the number
of antennas which have successfully pointed up, and lists those antennas
which have not. If multiple subarrays are pointing up at the same time,
the output from the various subarrays will be interleaved in some random
fashion.
The file emailed to the observer (if requested) is more substantial,
and more helpful. The following gives part of the pointing output from
a 7mm run. The content and format may change significantly with time. Lines
beginning with *** are comments -- they do not appear in the actual listing.
Note that primary and secondary pointing results are not
distinguished in this listing; you have either to compare the reported
time with the OBSERVE file, or to check the observing band.
file: mrupen1.ptg.96.12.20
-----------------------------------------------------------------
sub 1
The file to be viewed is: *BOSS0!H0/PTG_1/96_355_06:44/OFFSETS
Continue ? (Y or N):
y
Dts Source B Crmode
//OFS1 96 355 06:46 0112-017 X Continuum
*** OFS1: year, DOY, time, source, obs. band, and obs. mode (cont./line)
Date LST LST
//OFS2 96DEC20 06:46 1.449635 5h32m13.09s
Dts Wndspd Wnddir Temp Barom Dew Pnt
//WX1 96 355 06:46 2.18 255.78 -5.66 790.64 -15.31
*** WX1: reliable weather information from the control building.
*** The most interesting number is the windspeed [m/s], here measured
*** during the central pointing.
Dts Blk Ball Wht Ball Diff Wspmax Wsprms
//WX2 96 355 06:46 -9.81 -12.82 3.15 2.50 0.2750
*** WX2: more weather information from the control building.
*** The max. wind speed and wind speed rms [m/s] are determined over the
*** entire pointing scan.
//SUB 1 OBSERVE FILE: 147K432A
//SUB 2 DATA FILE: *BOSS0!H0/PTG_1/96_355_06:44
//SUB 3 SUBARRAY #: 1
//SUB 4 SUBN: 13
*** SUBN: number of antennas in subarray
//SUB 5 SUBIDS1: 3 4 6 8 11 12 13 14 16 20 22 25 27 -1
//SUB 6 SUBIDS2: -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
//SUB 7 REFN: 0
*** REFN: VLBI reference antenna(s). Not important for pointing scans.
//SUB 8 REFIDS1: -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
//SUB 9 REFIDS2: -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
//SUB 10 GOODN: 13
*** GOODN: number of antennas giving good PTG solutions
//SUB 11 GDIDS1: 3 4 6 8 11 12 13 14 16 20 22 25 27 -1
//SUB 12 GDIDS2: -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
//SUB 13 BADN: 0
*** BADN: number of antennas giving bad PTG solutions
//SUB 14 BADIDS1: -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
//SUB 15 BADIDS2: -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
//SUB 16 ANTNR: 3 AZ: -106d 23m 12.7s EL: 20d 13m 7.0s
*** ANTR: the Az and El of a representative antenna (here, #3).
//SUB 17 SINA: -0.9594 COSA: -0.2821 SINH: 0.3456 ABS(COSH): 0.9384
*** SINA etc: sines and cosines of that Az and El
Offset Records for dts = 96 355 06:46
# Elpt Elbm Azpt Azbm Power O S W Wsp1 Wsp2
3 R 0.334 1.100 -0.030 1.058 0.096 F F R 0.00 0.58
3 L 0.006 1.122 0.002 1.075 0.096 F F R 0.00 0.58
4 R 0.129 1.089 -0.056 1.072 0.096 F F R 1.50 -0.04
4 L -0.163 1.102 -0.134 1.096 0.097 F F R 1.50 -0.04
6 R 0.168 1.096 0.019 1.080 0.121 F F R -0.38 -0.34
6 L -0.158 1.061 0.037 1.081 0.119 F F R -0.38 -0.34
8 R 0.351 1.099 0.100 1.089 0.095 F F R 0.98 0.74
8 L 0.087 1.082 0.051 1.065 0.096 F F R 0.98 0.74
11 R 0.197 1.070 0.072 1.072 0.095 F F R 0.00 0.00
11 L -0.143 1.074 0.043 1.064 0.095 F F R 0.00 0.00
12 R -0.193 1.096 -0.061 1.095 0.095 F F R 1.50 5.16
12 L -0.493 1.081 -0.132 1.098 0.095 F F R 1.50 5.16
13 R -0.376 1.097 -0.003 1.076 0.095 F F R -0.02 2.79
13 L -0.678 1.071 -0.002 1.071 0.092 F F R -0.02 2.79
14 R -0.056 1.066 0.031 1.114 0.097 F F R 0.00 0.00
14 L -0.392 1.080 0.023 1.111 0.095 F F R 0.00 0.00
16 R 0.314 1.082 0.095 1.107 0.094 F F R 0.04 0.04
16 L 0.028 1.085 0.055 1.109 0.095 F F R 0.04 0.04
20 R 0.063 1.077 0.089 1.100 0.094 F F R 0.00 -0.04
20 L -0.226 1.090 0.050 1.083 0.093 F F R 0.00 -0.04
22 R 0.379 1.086 -0.107 1.078 0.096 F F R 0.07 4.00
22 L 0.139 1.088 -0.157 1.092 0.096 F F R 0.07 4.00
25 R 0.327 1.068 0.005 1.081 0.095 F F R 2.59 2.41
25 L 0.013 1.060 -0.070 1.078 0.095 F F R 2.59 2.41
27 R 0.077 1.084 0.049 1.100 0.094 F F R 0.51 0.00
27 L -0.215 1.088 -0.054 1.078 0.097 F F R 0.51 0.00
*** Fields are: antenna #; pol'n;
*** pointing offset in elevation [arcmin];
*** beam size in elev. [normalized; must be between 0.8 and 1.25
*** to be accepted as believable];
*** pointing offset in azimuth [arcmin];
*** beam size in azimuth [normalized; must be between 0.8 and 1.25
*** to be accepted as believable];
*** a number proportional to the amplitude;
*** three flags: Over the top?; Shadowed?; Wrap (left/right);
*** Wind SPeed at the antenna [m/s] (highly inaccurate) for each of two
*** anemometers at each telescope.
*** Antennas with failed pointing solutions will not appear in this
*** list at all.
End of offset records for this determination.
etc.